Quoted:
Many departments that allow rifles don't allow some "custom built AR with a BCM upper". Even if you buy yourself, you get a choice of a few manufacturers and you get to leave your gun the fuck alone. Some are even restricted to one specific model from one specific manufacturer. That sounds like the case in Little Rock.
And if it's used in a shooting, it's gone for a while. Would you rather tie up your gun or somebody else's gun?
My local sheriff rules for deputy-owned rifles as near as I can recall are:
Must be new purchase, No used, previously owned or parts builds. Must be purchased from one of two gun shops. Receipt is kept in deputy's file jacket.
Must be Bushmaster or Colt.
Only non-magnifying optics from certain brands allowed, black GI issue sling only. Collapsible stock allowed, but only if original to the gun. NO aftermarket parts. No lights.
Mags restricted to 20 rds, no polymer mags.
Ammo is dept issue only, IIRC, 55gr Federal softpoint. Officer held to a maximum 60 rounds.
It's all micro-managing BS.